Method for depositing thin tungsten film with low resistivity and robust micro-adhesion characteristics

ABSTRACT

Methods of forming low resistivity tungsten films with good uniformity and good adhesion to the underlying layer are provided. The methods involve forming a tungsten nucleation layer using a pulsed nucleation layer process at low temperature and then treating the deposited nucleation layer prior to depositing the bulk tungsten fill. The treatment operation lowers resistivity of the deposited tungsten film. In certain embodiments, the depositing the nucleation layer involves a boron-based chemistry in the absence of hydrogen. Also in certain embodiments, the treatment operations involve exposing the nucleation layer to alternating cycles of a reducing agent and a tungsten-containing precursor. The methods are useful for depositing films in high aspect ratio and/or narrow features. The films exhibit low resistivity at narrow line widths and excellent step coverage.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit under 35 U.S.C. 119(e) of U.S.Provisional Application No. 61/061,078, filed Jun. 12, 2008. Thisapplication is incorporated by reference herein in its entireties andfor all purposes.

FIELD OF INVENTION

This invention relates to methods for preparing tungsten films.Embodiments of the invention are useful for integrated circuitapplications that require thin tungsten films having low electricalresistivity with good uniformity and good adhesion.

BACKGROUND

The deposition of tungsten films using chemical vapor deposition (CVD)techniques is an integral part of many semiconductor fabricationprocesses. Tungsten films may be used as low resistivity electricalconnections in the form of horizontal interconnects, vias betweenadjacent metal layers, and contacts between a first metal layer and thedevices on the silicon substrate. In a conventional tungsten depositionprocess, the wafer is heated to the process temperature in a vacuumchamber, and then a very thin portion of tungsten film, which serves asa seed or nucleation layer, is deposited. Thereafter, the remainder ofthe tungsten film (the bulk layer) is deposited on the nucleation layer.Conventionally, the tungsten bulk layer is formed by the reduction oftungsten hexafluoride (WF₆) with hydrogen (H₂) on the growing tungstenlayer. The tungsten bulk layer is generally deposited more rapidly thanthe nucleation layer, but cannot be produced easily and reliably withoutfirst forming the nucleation layer.

SUMMARY OF INVENTION

Methods of forming low resistivity tungsten films with good uniformityand good adhesion to the underlying layer are provided. The methodsinvolve forming a tungsten nucleation layer using a pulsed nucleationlayer process at low temperature and then treating the depositednucleation layer prior to depositing the bulk tungsten fill. Thetreatment operation lowers resistivity of the deposited tungsten film.In certain embodiments, depositing the nucleation layer involves aboron-based chemistry in the absence of hydrogen. Also in certainembodiments, the treatment operations involve exposing the nucleationlayer to alternating cycles of a reducing agent and atungsten-containing precursor. The methods are useful for depositingfilms in high aspect ratio and/or narrow features. The films exhibit lowresistivity at narrow line widths and excellent step coverage.

These and other features and advantages of the invention will bedescribed in more detail below with reference to the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description can be more fully understood whenconsidered in conjunction with the drawings in which:

FIG. 1 shows simple cross-sectional diagrams of tungsten deposition inlow and high aspect ratio features.

FIG. 2 is a process flow sheet showing relevant operations of methodsaccording to various embodiments of the present invention.

FIGS. 3 a and 3 b are graphs representing reducing agent pulses andinterval times of the low resistivity treatment operations according tovarious embodiments of the invention.

FIGS. 4 a and 4 b are process flow sheets showing relevant operations ofmethods according to various embodiments of the present invention.

FIG. 5 is a simple cross-sectional diagram of a film stack including atitanium adhesion layer together with a tungsten nucleation layer and atungsten bulk layer formed in accordance with this invention.

FIG. 6 shows XRD spectra of a conventional nucleation layer and a layerformed according to an embodiment of the invention.

FIG. 7 is a block diagram of a processing system suitable for conductingtungsten deposition process in accordance with embodiments of theinvention.

DETAILED DESCRIPTION

Introduction

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention,which pertains to forming thin tungsten films. The methods involvepulsed nucleation layer (PNL) deposition techniques, which will bedescribed in detail below. Modifications, adaptations or variations ofspecific methods and or structures shown and discussed herein will beapparent to those skilled in the art and are within the scope of thisinvention.

In a PNL technique, pulses of the reducing agent, purge gases, andtungsten-containing precursors are sequentially injected into and purgedfrom the reaction chamber. The process is repeated in a cyclical fashionuntil the desired thickness is achieved. PNL is similar to atomic layerdeposition techniques reported in the literature. PNL is generallydistinguished from atomic layer deposition (ALD) by its higher operatingpressure range (greater than 1 Torr) and its higher growth rate percycle (greater than 1 monolayer film growth per cycle). In the contextof this invention, PNL broadly embodies any cyclical process ofsequentially adding reactants for reaction on a semiconductor substrate.Thus, the concept embodies techniques conventionally referred to as ALD.Additional discussion regarding PNL type processes can be found in U.S.Pat. Nos. 6,635,965, 6,844,258, 7,005,372 and 7,141,494 as well as inU.S. patent applications Ser. No. 11/265,531, incorporated herein byreference.

The present invention involves forming a tungsten film by way of atungsten nucleation layer. In general, a nucleation layer is a thinconformal layer which serves to facilitate the subsequent formation of abulk material thereon. The nucleation layer may be formed using one ormore PNL cycles. The methods described herein provide nucleation layersthat are very thin yet sufficient for good plugfill, have lowresistivity and exhibit good micro-adhesion. The methods are especiallyuseful for depositing tungsten in high aspect ratio and small features.

As features become smaller, the tungsten (W) contact or line resistanceincreases due to scattering effects in the thinner W film. Whileefficient tungsten deposition processes require tungsten nucleationlayers, these layers typically have higher electrical resistivities thanthe bulk tungsten layers. Thus, to keep the electrical resistance of theoverall tungsten film (tungsten nucleation layer and bulk tungsten) low,the tungsten nucleation layer should be kept as thin as possible. Asimplified equation describing the total resistance of a tungsten layeris:

R _(total) =R _(bulk) +R _(nucleation)=ρ_(bulk)(L _(bulk)/A)+ρ_(nucleation)(L _(nucleation) /A)

This is shown in the above simplified equation of total resistance,R_(total), where ρ is the resistivity of the material, L is the lengthof the layer in the direction of the current flow and A is thecross-sectional area perpendicular to current flow. (It should be notedthat certain contributions to the total resistance are neglected in theabove equation for the sake of explanation). Resistivity is an intrinsicproperty of a material and a measurement of a material's resistance tothe movement of charge through the material. The resistivity of amaterial affects the electrical operation of an integrated circuit. Lowresistivity tungsten films minimize power losses and overheating inintegrated circuit designs. Because the ρ_(nucleation)>ρ_(bulk), thethickness of the nucleation layer should be minimized to keep the totalresistance as low as possible. On the other hand, the tungstennucleation should be sufficiently thick to fully cover the underlyingsubstrate to support high quality bulk deposition. To achieve an optimalthickness, the tungsten nucleation layer may be formed in one or morePNL deposition cycles.

For narrow width and/or high aspect ratio and thin features, obtainingthin nucleation layers is even more critical. FIG. 1 shows a relativelylow aspect ratio feature 101 is shown in comparison to a relatively highaspect ratio feature 103. (These features are not drawn to scale but toillustrate the qualitative difference between nucleation layers in highand low aspect ratio features). Here, the thickness t is the same forboth features, but the width W2 of feature 103 is much less than widthW1 of feature 101 and the nucleation layer takes up a significantlyhigher percentage of the total volume of the feature. As a result, thenucleation layer has a much high relative contribution to the overallresistance of the feature. Thus, it becomes important to reduce thenucleation layer thickness (for example from a 50 A film to <30 A) forsmall features (for example a feature having a 10:1 aspect ratio or 400A opening) in order to reduce the overall stack resistivity.

In addition to providing tungsten films having low resistivity, themethods described herein provide films having good uniformity andadhesion to the underlying material. In certain embodiments, the methodsprovide good micro-adhesion as well as macroscopic adhesion. Macrosopicadhesion may be measured by a scribe/tape test. In a scribe/tape test,the tungsten film is scribed with a diamond cutter, tape is placed overthe scribed area, and then the tape is pulled off. “Pass” for adhesionindicates that the tungsten film remained on the titanium nitridebarrier layer after a scribe/tape test, whereas “Fail” indicates thatthe tape removed portions of the tungsten film. Poor micro-adhesionresults in micron-scale peeling of the deposited tungsten film. A filmmay have acceptable macro-scale adhesion, remaining on the underlyinglayer in a scribe/test, while still exhibiting micro-peeling.

The methods involve forming a tungsten nucleation layer in a featureusing a pulsed nucleation layer process at low temperature and thentreating the deposited nucleation layer prior to depositing the bulktungsten fill. FIG. 2 presents a process flow sheet showing an overviewof operations according to certain embodiments. Initially, a substrateis provided and positioned in a reaction chamber as indicated by aprocess block 201. As mentioned previously, in many embodiments thesubstrate is a partially fabricated electronic device (e.g., a partiallyfabricated integrated circuit). Specific applications of the inventionare described further below. The substrate contains a feature that has ahigh aspect ratio and/or narrow width. According to various embodiments,high aspect ratios range from 5:1-30:1. In certain embodiments, theaspect ratio is at least 10:1 or 20:1. Features having widths as narrowas 300-400 Angstroms also benefit from this process. In some cases, boththe feature has both a high aspect ratio and a narrow width, butfeatures having only one of these geometric characteristics benefit fromthe processes. For example, in one embodiment, a low resistivitytungsten layer is deposited in a feature having a width around 500Angstrom and aspect ratio of about 30:1. In certain embodiments, themethods may also be advantageously used to deposit low resistivitytungsten film on planar surfaces and surfaces having lower aspect ratiofeatures and wider features.

Next, as indicated by a process block 203, a low temperature pulsednucleation layer (PNL) process is performed to deposit a tungstennucleation layer. Depositing tungsten nucleation layer using a PNLprocess involves exposing the substrate to alternating pulses of areducing agent and a tungsten-containing precursor, such as WF₆. Lowtemperature tungsten nucleation layer processes to deposit conformalnucleation layers are described in U.S. patent application Ser. No.11/265,531, filed Nov. 1, 2005, incorporated by reference herein in itsentirety and for all purposes. Substrate temperature is low—below about350 C, for example between about 250 and 350 C or 250 and 325 C. Incertain embodiments, the temperature is around 300 C. Above-referencedapplication Ser. No. 11/265,531 describes sequences of reducingagent/tungsten-containing precursor pulses that result may be used todeposit low resistivity film. According to various embodiments,boron-containing (e.g., diborane) and non-boron-containing (e.g.,silane) reducing agents are used to deposit the nucleation layers. Also,in certain embodiments, nucleation layer deposition includes one or morehigh temperature (e.g., 395° C.) PNL cycles after the low temperaturecycles. In certain embodiments, methods for depositing tungstennucleation layers in very small/high aspect ratio features as describedin U.S. patent application Ser. No. 12/030,645, filed Feb. 13, 2008,incorporated by reference herein in its entirety and for all purposes,are used to deposit the nucleation layer. These methods involve usingPNL cycles of a boron-containing reducing agent and atungsten-containing precursor with no hydrogen in the background todeposit very thin tungsten nucleation layers (e.g., about 12 Angstroms)in these features that have good step coverage. In certain embodimentsfollowing these methods, diborane or (another borane or boron-containingreducing agent) is the only reducing agent used during deposition of thenucleation layer.

Referring back to FIG. 2, the next operation 205 involves a highertemperature treatment process to lower resistivity. FIGS. 3 a and 3 bare graphs showing examples of treatments that may be performed. FIG. 3a shows an example of a treatment process such as described in such asthat described in U.S. patent application Ser. No. 11/951,236, filedDec. 5, 2007, incorporated by reference herein in its entirety and forall purposes. The treatment process described therein involves exposingthe deposited nucleation layer to multiple pulses of a reducing agent(without intervening pulses of another reactive compound). In thefigure, diborane is depicted as the reducing agent, though otherreducing agents may be used. The treatment lowers resistivity, whileproviding good adhesion and resistance non-uniformity. Notably, usingmultiple reducing agent pulses is shown to provide significantlyimproved resistivity and uniformity than using a single pulse—even withthe same overall exposure time. However, too many pulses lead to pooradhesion of the eventual tungsten film to the underlying layer. Asdiscussed in the Ser. No. 11/951,236 application, an optimal number ofpulses, e.g., between 2-8, is used to obtain low resistivity, lownon-uniformity and acceptable adhesion.

FIG. 3 b shows another example of a treatment process in which thesubstrate having the nucleation layer deposited thereon is exposed tomultiple cycles of alternating reducing agent and a tungsten-containingprecursor pulses. Diborane, B₂H₆, and tungsten hexafluoride, WF₆, areshown as the reducing agent and tungsten-containing precursor,respectively, though certain embodiments may use other compounds.

Alternating pulses of a reducing agent and tungsten-containing precursorare also used to deposit the tungsten nucleation layer, but in thetreatment operation, typically substantially no tungsten is deposited.It has been found that in certain cases, using such a treatmentoperation provides film with fewer defects than the multiple pulsetreatment show illustrated in FIG. 3 a. In particular, alternating B₂H₆and WF₆ has been shown to substantially reduce or eliminate instances ofmicro-peeling—micron-scale, localized areas of peeling of the tungstenbulk layer from the underlying surface. Without being bound by aparticular theory, it is believed that is because the WF₆ or othertungsten precursor scavenges residual reducing agent on the film.

As indicated in FIG. 2, the treatment process is performed at a highertemperature than the nucleation layer deposition. Temperatures rangefrom 375 C to 415 C, e.g., about 395 C. Transitioning from nucleationlayer deposition to this treatment operation may involve heating thesubstrate to between about 350 C and 415 C, or in certain embodiments toabout 375 C to 415 C and allowing it to stabilize before exposing thenucleation layer to a plurality of reducing agent or reducingagent/tungsten-containing precursor pulses in process. As indicated incertain embodiments the substrate temperature is about 395 C. Lowertemperatures would require longer pulse times to achieve equivalenttreatment effect.

Examples of gas flow rates of the reducing agent (andtungsten-containing precursor if used) during a pulse is between about100 to 500 sccm. The pulse time may vary from between about 0.5 to 5seconds, e.g., between about 1 to 2 seconds. The interval time betweeneach pulse typically varies between about 2 to 5 seconds. When atungsten-containing precursor is used, as depicted in FIG. 3 b, thepulse time should be short enough to ensure that no or substantially notungsten deposits. (In certain embodiments, some small amount oftungsten, e.g., about or less than an atomic layer may be depositedduring the treatment). In certain embodiments, the reducing agent andtungsten-containing precursor pulses may be as short as less than 1second. In one example B₂H₆ is pulsed for 1 second, followed by a 1second purge, followed by a WF₆ pulse of 1 second, followed by a 2.5second purge. This cycle is then repeated four times.

For these operating conditions, the number of reducing agent pulses (asin FIG. 3 a) and or reducing agent/tungsten precursor cycles (as in FIG.3 b) is typically between 2 and 8. Five pulses or cycles are used inparticular embodiments. Chamber pressure can vary broadly during themulti-pulse reducing agent treatment, between about 2 and 100 Torr, andmore preferably between about 20 and 40 Torr. These parameters are basedon 300 mm wafers and may need to be adjusted depending on the wafersize, particular processing equipment, particular reducing agent used,etc.

It has been found that depending on the pulse time, pulse dose, andinterval time, there exists an optimum number of pulses to use to obtainthe desired tungsten film properties. If too few pulses are used, theresistivity and sheet resistance uniformity of the tungsten film arepoor. If too many pulses are used, the resistivity and uniformity of thetungsten film are good, but adhesion is poor and micro-peelingincreases. In many embodiments, the optimum is between 2-8, though theoptimum number of pulses depends on the operating conditions used. Asignificantly higher number of pulses may be used for significantlydifferent processing conditions. Gas flow rate and/or pulse time may beidentical or may be varied from pulse to pulse.

Returning to FIG. 2, once the tungsten nucleation layer is treated, abulk tungsten layer is deposited in the feature in a process operation207. In many embodiments the bulk tungsten is deposited using a CVDprocess. CVD processes rapidly produce low resistivity films. Anysuitable CVD process may be used with any suitable tungsten-containingprecursor. In some embodiments the same tungsten-containing precursorused in the PNL processes for forming the tungsten nucleation layer isuse—typically one of WF₆, WCl₆ and W(CO)₆. Frequently, the CVD processis performed using a mixture of molecular hydrogen and one or more ofthese precursors. In other embodiments, the CVD process may employ atungsten precursor together with silane or a mixture of hydrogen andsilane or a mixture of hydrogen and borane (such as diborane). Non-CVDprocess can also be employed to form the bulk layer. These includeALD/PNL and physical vapor deposition (PVD).

The bulk tungsten can be deposited to any thickness. Tungsteninterconnect lines for integrated circuit applications may have a totalthickness (tungsten nucleation layer and bulk tungsten) of between about20 and 1,000 Angstroms. For a typical bit line, the total tungsten filmthickness typically is no more than about 600 Angstroms. The resultingtungsten film will preferably have a resistivity of no greater thanabout 30 μΩ-cm. Resisitivity depends on how much of the total thicknessis due to the nucleation layer. The resistivity for 600 A film(nucleation+CVD tungsten) deposited using the process described abovewith reference to FIG. 2, the resistivity for a 600 A film is less thanabout 14 μΩ-cm, and in certain cases less than about 11 μΩ-cm. Moreover,the film exhibits lower resistivity than film that is not treated. Afterthe tungsten film is deposited to a sufficient thickness, the processflow of FIG. 1 is complete.

FIG. 4 a is a process flow sheet showing a particular embodiment of theprocess depicted in FIG. 2. Here, as in FIG. 2, a substrate having ahigh aspect ratio and/or a narrow width is provided to a depositionchamber in an operation 401. A low temperature PNL process is thenperformed by exposing the substrate to alternating pulses of B₂H₆ andWF₆ in an operation 403. No hydrogen is present during deposition ofthis nucleation layer. In one example, B₂H₆ is pulsed for 2 seconds,followed by a 3 second purge, followed by a 0.5 second WF₆ pulse and a 3second purge. This repeated as necessary to deposit the nucleation layerconformally in the feature to the desired thickness. Using this lowtemperature PNL process, the nucleation layer may have thickness of lessthan about 15 Angstroms, e.g., 12 Angstroms, and still be sufficient forgood plugfill. Substrate temperature is then raised, e.g., from about300° C. to about 395° C., for the low resistivity treatment in anoperation 407. Other temperatures may be used; in certain embodiments,the temperature is raised at least 50° C. or at least 75° C. Thedeposited nucleation layer is then exposed to alternating B₂H₆ and WF₆pulses in the presence of hydrogen in an operation 409. As describedabove, there is typically no measurable amount of tungsten deposited inthis operation. The effect of this operation to lower resistivity of thetungsten plug. In certain embodiments, between 2 and 8 cycles, e.g., 5cycles are performed. After the multiple pulse treatment, a bulktungsten layer is then deposited in an operation 409. In multi-stationdeposition apparatuses, the nucleation layer may be deposited in a firststation, with the low resistivity treatment performed in one or moreadditional stations.

As discussed further below in the experimental section, processesaccording to the embodiment shown in FIG. 4 a, i.e., depositing a PNLnucleation layer without hydrogen running in the background and usingmultiple cycles of B₂H₆/WF₆ in the treatment operation, result in lowerresistivity, good adhesion and no or reduced micro-peeling, as comparedto processes that deposit the nucleation layer in the presence ofhydrogen and/or use reducing agent-only treatment operations.

Using a boron-based nucleation chemistry at relatively low temperature(e.g., 300° C.) in the absence of hydrogen and a boron-based resistivitytreatment at a higher temperature, as is done in certain embodiments ofthe methods described in FIGS. 2-4 results in films having excellentstep coverage and low resistivity. FIG. 6 shows XRD spectra for aconventional nucleation process (using silane as a reducing agent in thepresence of hydrogen) and diborane-based nucleation layer formed in theabsence of hydrogen. The conventional film shows peaks corresponding toW crystallinity, while the diborane-based process appears to beamorphous W. Without being bound to a particular theory, it is believedthat the amorphous nature of the film promotes conformal tungsten fillinside a trench or other feature. The absence of grain boundaries alsoprotects the underlying barrier layer from fluorine attack during asubsequent aggressive CVD reaction. As a result, the nucleation layeritself has a lower resistivity than nucleation layers deposited usingconventional PNL processes. Moreover, using pulses of B₂H₆ or pulses ofB₂H₆/WF₆ to treat the nucleation film promotes larger W grain growthduring CVD fill.

FIG. 4 b is a process flow sheet showing operations for anotherembodiment. A substrate is provided to a deposition chamber in anoperation 451. According to various embodiments, the substrate may haveat least on high aspect ratio/narrow feature, though as in thoseembodiments, the methods are not limited to such substrates. A lowtemperature PNL process is then performed to deposit a tungstennucleation layer in an operation 453. Unlike the process described inFIG. 4 a, hydrogen is run in the background. Depositing the nucleationlayer typically involves multiples cycles of alternating pulses of WF₆and one or more reducing agent. In one embodiment, depositing thenucleation layer involves a single cycle of B₂H₆ and WF₆ alternatingpulse followed by multiple cycles SiH₄ and WF₆. Substrate temperature isthen raised, e.g., from about 300° C. to about 395° C., for the lowresistivity treatment in an operation 457. Other temperatures may beused; in certain embodiments, the temperature is raised at least 50° C.or at least 75° C. The deposited nucleation layer is then exposed toalternating B₂H₆ and WF₆ pulses in the presence of hydrogen in anoperation 459. As described above, there is typically no measurableamount of tungsten deposited in this operation. The effect of thisoperation to lower resistivity of the tungsten plug. In certainembodiments, between 2 and 8 cycles, e.g., 5 cycles are performed. Afterthe multiple pulse treatment, a bulk tungsten layer is then deposited inan operation 459. In multi-station deposition apparatuses, thenucleation layer may be deposited in a first station, with the lowresistivity treatment performed in one or more additional stations.

As described further below with reference to Example 7, the processdescribed in FIG. 4 b has been shown to provide lower resistivity ascompared to a process that does not use multi-pulse treatment. Moreover,adhesion is improved over processes that use a multi-pulse treatment ofa boron-containing reducing agent with no intervening tungsten precursorpulses; those processes lower resistivity, but may have poor adhesion,e.g., as evidenced by peeling. As with the process in FIG. 4 a, themulti-pulse treatment described in operation 459 significantly reducesthe possibility of tungsten micro-peeling that may occur where multiplepulses of a boron-containing agent are used without an interveningtungsten precursor pulse (as in FIG. 3 a) to lower resistivity. Asindicated above, without being bound by a particular theory, it isbelieved that the introduction of the WF₆ pulses between B₂H₆ pulseshelps scavenge unreacted B₂H₆, which otherwise promotes the onset ofmicropeeling, from the film surface. In one example of the multi-pulseboron-containing compound/tungsten precursor treatment operationsdescribed above, B₂H₆ is pulsed for 1 second, followed by a 1 secondpurge, followed by a 1 second WF₆ pulse, followed by a 2.5 second purge.The process is then repeated four times. In a particular example, theB₂H₆ flow rate is 300 sccm and the WF₆ flow rate is 100 sccm.

According to various embodiments, the process may be used to providetungsten films having a resistivity at 600 Angstroms of no more thanabout 14 μΩ-cm or in certain embodiments, no more than about 11 μΩ-cm.The films may also have a resistance non-uniformity of less than about5%.

Experimental

The following examples are provided to further illustrate aspects andadvantages of the present invention. These examples are provided toexemplify and more clearly illustrate aspects of the present inventionand are in no way intended to be limiting.

EXAMPLE 1

A W nucleation layer was formed in features having an AR of 8.5:1 and atop opening was 0.14 μm at 300° C. using tungsten nucleation layerdeposition sequences shown in the table below. Nucleation layers ofabout 42 Å for process A, 25 Å for process C and 35 Å for process B weredeposited. Treatment operations were then performed using sequencesshown below at 395° C. (Note that for process A, the ‘treatment’involved a B2H6/WF6 cycle having longer pulse duration; tungsten filmwas deposited during this step.) Note that process is in accordance withthe embodiments depicted in FIG. 4 a. A bulk tungsten layer was thendeposited on each nucleation layer. Resistivity at 600 Angstroms andresistance non-uniformity at 3 mm edge exclusion were measured. Thefilms were also examined for areas of micro-peeling. Process conditionsand results are shown below in Table 1

TABLE 1 Resistivity at Nucleation 600 Layer Angstroms Micro- DepositionTreatment (μΩ- 4 mm EE peeling Process Sequence Sequence cm) Rs % NUobserved? A 1 B2H6/WF6 1 13 about 9% No cycle + 4 B2H6/ SH4/WF6 WF6cycles (in cycle - presence of with H2) tungsten deposition B 1 B2H6/WF65 B2H6 10 about 3% Yes cycle + 3 pulse SH4/WF6 cycles (in presence ofH2) C 5 B2H6/WF6 5 10.7 about 3% No cycles (no B2H6/ H2) WF6 - notungsten deposition

Processes B and C, which have multiple pulse treatment operations,provide improved resistivity over process A. Process C, which uses nohydrogen in the tungsten nucleation layer deposition and uses WF6 pulsesin the treatment operation provides the resistivity benefits seen withprocess B, but without any micro-peeling.

EXAMPLE 2

A W nucleation layer was formed on semiconductor substrates (planar) at300° C. using tungsten nucleation layer deposition sequences shown inthe table below. Nucleation layer thicknesses of about 35 Å for ProcessD, about 25 Å for processes E and F were deposited. Process D used asingle B2H6/WF6 cycles followed by three SiH4/WF6 cycles in the presenceof H2; processes E and F used a low resistivity tungsten depositionprocess without any hydrogen. Low resistivity treatment operations werethen performed using 5 cycles of the sequences shown below at 395° C.Process D used pulses of B2H6 (no intervening pulses); processes E and Fboth used alternating B2H6 and WF6 pulses. Processes E and F wereperformed in accordance with the embodiments depicted in FIG. 4 a. Abulk tungsten layer was then deposited on each nucleation layer.Resistivity at 600 Angstroms and resistance non-uniformity at 4 mm edgeexclusion were measured. For each process, conditions were optimized tominimize micro-peeling and defects. The magnitude of defects for eachprocess was the same. Process conditions and results are shown below inTable 2:

TABLE 2 Nucleation Treatment - 5 cycles Resistivity Layer B2H6 WF6 atDeposition flowrate/ flowrate/ 4 mm EE 600 Å Process Sequence pulse timepulse time Rs % NU (μΩ-cm) D 1 B2H6/WF6 200 sccm/ 0/0 5.97 13.59 cycle +3 1 sec SH4/WF6 cycles (in presence of H2) E 5 B2H6/WF6 250 sccm/ 0/01.83 10.09 cycles (no 0.5 sec H2) F 5 B2H6/WF6 200 sccm/ 0/0 1.65 11.42cycles (no 0.5 sec H2)

As indicated above, the quality of all films as measured by numberdefects were about the same. Optimized for fewer defects, processes Dand F show significantly improved resistivity (10.09 and 11.42 μΩ-cm ascompared to 13.59 μΩ-cm) and resistance non-uniformity (1.83% and 1.65%as compared to 5.97%).

EXAMPLE 3

Similarly, when tuned for low resistivity, processes as shown in FIG. 4a result in lower particle counts and micro-peeling for similarresistivities:

TABLE 3 Nucleation Treatment - 5 cycles Resistivity Layer B2H6 WF6 atMicro- Deposition flowrate/ flowrate/ 600 Å peeling Process Sequencepulse time pulse time (μΩ-cm) observed? G 1 B2H6/WF6 350 sccm/ 0/0 9.83Yes cycle + 3 1 sec SH4/WF6 cycles (in presence of H2) H 5 B2H6/WF6 250sccm/ 0/0 9.76 No or cycles (no 1 sec reduced** H2) (**For particularsplit shown here, micropeeling was not measured; however from otherexperiments, it was shown that Process H results in no or reducedmicropeeling compared to Process G.)

EXAMPLE 4

Various processes according to the embodiment shown in FIG. 4 a wereused to deposit and treat tungsten nucleation layers. In particular, thenucleation layer was deposited according to the sequence described inTables 1 and 2 for processes C, E and F. Tungsten-containing precursorsand boron-containing reducing agent flow rates and pulse times werevaried in the following ranges:

-   Tungsten-containing precursor (WF₆) flow rate: 75-150 sccm;-   Tungsten-containing precursor (WF₆) pulse time 0.5-1.5 seconds;-   Boron-containing reducing agent (B₂H₆) flow rate: 200-300 sccm;-   Boron-containing reducing agent (B₂H₆) pulse time: 0.5-1 seconds.    Pulses were uniform for the treatment processes, i.e., the same WF₆    flow rate, WF₆ pulse time, B₂H₆ flow rate and B₂H₆ pulse time were    used for each of the multiple treatment pulses of a particular    process. CVD layers were deposited on each of the nucleation layers    and resistivity, resistance non-uniformity and particle counts were    examined. Based on the resulting experimental data, a prediction was    made optimizing the particle count, resistivity and resistance    non-uniformity. The predicted optimized process (WF₆ pulse of 0.5    seconds and 125 sccm; B₂H₆ pulse of 0.5 seconds and 270 sccm) was    then used to form a tungsten nucleation layer on which a CVD    tungsten film was deposited. This is a just an example of pulse    times and flow rates; depending on the particular process conditions    and desired results, others may be used.

EXAMPLE 5

The following processes were compared:

Process I

Nucleation layer formed by: B2H6/Ar purge/WF6/Ar purge (1 cycle)followed by SiH4/Ar purge/WF6/Ar purge (5 cycles) at 300° C. and 40 Torrin H2 ambient. Bulk fill by WF6 CVD with H2 reduction at 395° C.

Process J

Nucleation layer formed by: B2H6/Ar purge/WF6/Ar purge in H2 absence (5cycles). Low resistivity treatment by B2H6/Ar purge (6 cycles) at 395°C. in H2 ambient. Bulk fill by WF6 CVD with H2 reduction at 395° C.

FIG. 6 shows XRD spectra from each of the films; process I labeled asthe conventional PNL nucleation process and process J labeled asB2H6/WF6 no H2 process. As discussed above, the spectra indicates thatthe conventional film is crystalline and the nucleation layer formed bythe boron-based, no hydrogen process amorphous. Plugfill experiments on10:1 AR features show that for process I, a nucleation film of at least23 Å is required to achieve good plugfill step coverage. Insufficientnucleation layer near the feature bottom causes delay in the ensuingH2-WF6 CVD reaction and a void in the feature. However, for process J,excellent plugfill step coverage is achieved with as little as 12 μΩ-cmof nucleation film. It has also been found that the resistivity of thisnucleation film (55 μΩ-cm for 25 Å) is lower than that for the PNLnucleation film formed by process 1 (76 μΩ-cm for 25 Å).

For 500 Å film deposited on PVD TiN, W grain size on a blanket wafer isthree times larger using Process J over Process I.

EXAMPLE 6

To validate the effect of tungsten grain size differences on electricalperformance, line resistance measurements were performed on 90 nm (AR2:1) lines. 75 Å Ti and 120 Å CVD-TiN were used as liner and barrierrespectively. Four processes were used for this study as shown below inTable 4.

TABLE 4 Drop in median line resistance from conventional PNL NucleationTreatment (process 4) 1 B2H6/WF6 no H2 (5 B2H6 at 395° C. in H2 42%cycles) (6 cycles) 2 B2H6/WF6 no H2 (5 None 32% cycles) 3 B2H6/WF6 (1cycle) B2H6 at 395° C. in H2 22% followed by SiH4/ (6 cycles) WF6 (5cycles) 4 B2H6/WF6 (1 cycle) None followed by SiH4/ WF6 (5 cycles)

Compared to conventional PNL nucleation film, the boron-based nucleationfilm used in processes 1 and 2 results in reduced line resistivity dueto (i) larger in-trench W grain size resulting in less electronscattering at the grain boundaries (ii) lower resistivity of thenucleation film and (iii) higher percentage of the CVD W fill due tothinner nucleation. The low resistivity treatment used in processes 1and 3 also causes line resistivity reduction by promoting large graingrowth during CVD fill.

EXAMPLE 7

A W nucleation layer was formed in features having an AR of 8.5:1 and atop opening was 0.14 μm at 300° C. using tungsten nucleation layerdeposition sequences shown in the table below. Nucleation layers ofabout 40 Å for process A*, about 40 Å for process B* and about 40 Å forprocess K were deposited. (Processes A* and B* are the same processes Aand B shown in Table 1 of Example 1 in a different experiment).Treatment operations were then performed using sequences shown below at395° C. (Note that for process A*, the ‘treatment’ involved a B2H6/WF6cycle having longer pulse duration; tungsten film was deposited duringthis step.) A bulk tungsten layer was then deposited on each nucleationlayer. Resistivity at 600 Angstroms and resistance non-uniformity at 4mm edge exclusion were measured. The films were also examined for areasof micro-peeling. Process conditions and results are shown below inTable 5.

TABLE 5 Resistivity at Nucleation 600 Layer Angstroms Micro- DepositionTreatment (μΩ- 4 mm EE peeling Process Sequence Sequence cm) Rs % NUobserved? A* 1 B2H6/WF6 1 13 * Very little cycle + 4 B2H6/ or noneSH4/WF6 WF6 cycles (in cycle - presence of with H2) tungsten depositionB* 1 B2H6/WF6 5 B2H6 10 about 2% Yes cycle + 3 pulse SH4/WF6 cycles (inpresence of H2) K 1 B2H6/WF6 5 10.6 about 4% Very little cycle + 3 B2H6/SH4/WF6 WF6 - no cycles (in tungsten presence of deposition H2) *Nodata; prediction of 7-9%

Processes B* and K, which have multiple pulse treatment operations,provide improved resistivity over process A*. Process K, which uses WF6pulses in the treatment operation provides the resistivity benefits seenwith process B*, but without any micro-peeling.

Apparatus

The methods of the invention may be carried out in various types ofdeposition apparatus available from various vendors. Examples ofsuitable apparatus include a Novellus Concept-1 Altus, a Concept 2Altus, a Concept-2 ALTUS-S, a Concept 3 Altus deposition system, or anyof a variety of other commercially available CVD tools. In some cases,the process can be performed on multiple deposition stationssequentially. See, e.g., U.S. Pat. No. 6,143,082, which is incorporatedherein by reference for all purposes. In some embodiments, the pulsednucleation process is performed at a first station that is one of two,five or even more deposition stations positioned within a singledeposition chamber. Thus, the reducing gases and the tungsten-containinggases are alternately introduced to the surface of the semiconductorsubstrate, at the first station, using an individual gas supply systemthat creates a localized atmosphere at the substrate surface.

In one example, after a tungsten nucleation layer is deposited, thewafer is moved to a second station for part or all of a treatmentprocess and a new wafer is moved into place on the first station. Thewafers may be indexed from one station to the next to enable parallelwafer processing.

FIG. 7 is a block diagram of a processing system suitable for conductingtungsten thin film deposition processes in accordance with embodimentsof the invention. The system 700 includes a transfer module 703. Thetransfer module 703 provides a clean, pressurized environment tominimize the risk of contamination of substrates being processed as theyare moved between the various reactor modules. Mounted on the transfermodule 703 is a multi-station reactor 709 capable of performing PNLdeposition, multi-pulse treatment, and CVD according to embodiments ofthe invention. Chamber 709 may include multiple stations 711, 713, 715,and 717 that may sequentially perform these operations. For example,chamber 709 could be configured such that station 711 performs PNLdeposition, station 713 performs multi-pulse treatment, and stations 715and 717 perform CVD.

Also mounted on the transfer module 703 may be one or more single ormulti-station modules 707 capable of performing plasma or chemical(non-plasma) pre-cleans. The module may also be used for various othertreatments, e.g., post liner tungsten nitride treatments. The system 700also includes one or more (in this case two) wafer source modules 701where wafers are stored before and after processing. An atmosphericrobot (not shown) in the atmospheric transfer chamber 719 first removeswafers from the source modules 701 to loadlocks 721. A wafer transferdevice (generally a robot arm unit) in the transfer module 703 moves thewafers from loadlocks 721 to and among the modules mounted on thetransfer module 703.

In certain embodiments, a system controller is employed to controlprocess conditions during deposition. The controller will typicallyinclude one or more memory devices and one or more processors. Theprocessor may include a CPU or computer, analog and/or digitalinput/output connections, stepper motor controller boards, etc.

The controller may control all of the activities of the depositionapparatus. The system controller executes system control softwareincluding sets of instructions for controlling the timing, mixture ofgases, chamber pressure, chamber temperature, wafer temperature, RFpower levels, wafer chuck or pedestal position, and other parameters ofa particular process. Other computer programs stored on memory devicesassociated with the controller may be employed in some embodiments.

Typically there will be a user interface associated with the controller.The user interface may include a display screen, graphical softwaredisplays of the apparatus and/or process conditions, and user inputdevices such as pointing devices, keyboards, touch screens, microphones,etc.

The computer program code for controlling the deposition and otherprocesses in a process sequence can be written in any conventionalcomputer readable programming language: for example, assembly language,C, C++, Pascal, Fortran or others. Compiled object code or script isexecuted by the processor to perform the tasks identified in theprogram.

The controller parameters relate to process conditions such as, forexample, process gas composition and flow rates, temperature, pressure,plasma conditions such as RF power levels and the low frequency RFfrequency, cooling gas pressure, and chamber wall temperature. Theseparameters are provided to the user in the form of a recipe, and may beentered utilizing the user interface.

Signals for monitoring the process may be provided by analog and/ordigital input connections of the system controller. The signals forcontrolling the process are output on the analog and digital outputconnections of the deposition apparatus.

The system software may be designed or configured in many differentways. For example, various chamber component subroutines or controlobjects may be written to control operation of the chamber componentsnecessary to carry out the inventive deposition processes. Examples ofprograms or sections of programs for this purpose include substratepositioning code, process gas control code, pressure control code,heater control code, and plasma control code.

A substrate positioning program may include program code for controllingchamber components that are used to load the substrate onto a pedestalor chuck and to control the spacing between the substrate and otherparts of the chamber such as a gas inlet and/or target. A process gascontrol program may include code for controlling gas composition andflow rates and optionally for flowing gas into the chamber prior todeposition in order to stabilize the pressure in the chamber. A pressurecontrol program may include code for controlling the pressure in thechamber by regulating, e.g., a throttle valve in the exhaust system ofthe chamber. A heater control program may include code for controllingthe current to a heating unit that is used to heat the substrate.Alternatively, the heater control program may control delivery of a heattransfer gas such as helium to the wafer chuck.

Examples of chamber sensors that may be monitored during depositioninclude mass flow controllers, pressure sensors such as manometers, andthermocouples located in pedestal or chuck. Appropriately programmedfeedback and control algorithms may be used with data from these sensorsto maintain desired process conditions.

The foregoing describes implementation of embodiments of the inventionin a single or multi-chamber semiconductor processing tool.

Applications

The present invention may be used to deposit thin, low resistivitytungsten layers for many different applications. One preferredapplication is for interconnects in integrated circuits such as memorychips and microprocessors. Interconnects are current lines found on asingle metallization layer and are generally long thin flat structures.These may be formed by a blanket deposition of a tungsten layer (by aprocess as described above), followed by a patterning operation thatdefines the location of current carrying tungsten lines and removal ofthe tungsten from regions outside the tungsten lines.

A primary example of an interconnect application is a bit line in amemory chip. Of course, the invention is not limited to interconnectapplications and extends to vias, contacts and other tungsten structurescommonly found in electronic devices. In general, the invention findsapplication in any environment where thin, low-resistivity tungstenlayers are required.

Another parameter of interest for many applications is a relatively lowroughness of the ultimately deposited tungsten layer. Preferably, theroughness of the tungsten layer is not greater than about 10% of thetotal thickness of the deposited tungsten layer, and more preferably notgreater than about 5% of the total thickness of the deposited tungstenlayer. The roughness of a tungsten layer can be measured by varioustechniques such as atomic force microscopy.

FIG. 5 is a cross-section illustrations of a film stack that can beformed using methods of the invention. The film stack may representinterconnect applications as described previously. The film stack ofFIG. 5 is formed in an underlying substrate having a feature in whichtungsten is to be deposited. The feature may be a single component ormore commonly a complex multi-feature structure having variousconductive, insulating, and semiconductor components. For example, thesubstrate may have a top layer comprising silicon or a dielectric suchas silicon dioxide. Contacting the substrate is, in the following order,a titanium layer 503, a titanium nitride layer 505, a tungstennucleation layer 507 (formed in accordance with this invention) and atungsten bulk layer 509. Titanium layer 503 is typically deposited by aCVD process which provides reasonably good adhesion to the underlyingsubstrate 501. Titanium nitride layer 505 is typically deposited usingCVD or PVD methods and is used to protect the underlying titanium and/orsilicon from exposure to tungsten hexafluoride (WF₆) during subsequenttungsten deposition. It has been found that WF₆ reacts very aggressivelyand sometimes explosively with titanium. Tungsten nucleation layer 507and tungsten bulk layer 509 are formed in accordance with the methods ofthe present invention as described above. In interconnect applicationsas described above, layers 503, 505, 507 and 509 are all etched to forminterconnect lines. In another embodiment, a tungsten nitride layer isemployed instead of the Ti/TiN layer.

Other Embodiments

While this invention has been described in terms of several embodiments,there are alterations, modifications, permutations, and substituteequivalents, which fall within the scope of this invention. It shouldalso be noted that there are many alternative ways of implementing themethods and apparatuses of the present invention. It is thereforeintended that the following appended claims be interpreted as includingall such alterations, modifications, permutations, and substituteequivalents as fall within the true spirit and scope of the presentinvention.

1. A method of forming a tungsten film on a substrate in a reactionchamber, the method comprising: exposing the substrate to alternatingpulses of a tungsten-containing precursor and a reducing agent tothereby deposit a tungsten nucleation layer on the substrate; performinga treatment operation on the deposited tungsten nucleation layer,wherein the treatment operation comprises exposing the tungstennucleation layer to alternating pulses of a boron-containing compoundand a tungsten-containing precursor wherein substantially no tungsten isdeposited on the tungsten nucleation layer during said treatmentoperation; and depositing a tungsten bulk layer over the treatedtungsten nucleation layer to form the tungsten film.
 2. The method ofclaim 1 wherein treating the deposited tungsten nucleation layer lowersresistivity of deposited tungsten film.
 3. The method of claim 1 whereintreating the deposited tungsten nucleation layer lowers the resistivityof the nucleation film.
 4. The method of claim 1 wherein the treatmentoperation is performed at a substrate temperature of between about350-415° C.
 5. The method of claim 1 wherein the treatment operation isperformed at a substrate temperature of about 395° C.
 6. The method ofclaim 1 wherein deposition of the tungsten nucleation layer is performedat a substrate temperature of between about 250-310° C. and thetreatment operation is performed at a temperature of between about350-415° C.
 7. The method of claim 1 wherein the treatment operationcomprises between 2 and 8 alternating pulses of a boron-containingcompound and a tungsten-containing precursor.
 8. The method of claim 1wherein deposition of the tungsten nucleation layer is performed at asubstrate temperature of between about 250-350° C.
 9. The method ofclaim 1 wherein deposition of the tungsten nucleation layer is performedat a substrate temperature of between about 300° C.
 10. The method ofclaim 1 wherein deposition of the tungsten nucleation layer is performedat a substrate temperature of between about 250-325° C. whereinsubstantially no hydrogen is flowed during or between the pulses. 11.The method of claim 10 wherein the treatment operation is performed at atemperature of between about 350-415° C. with hydrogen flowing in thebackground.
 12. The method of claim 1 wherein transition from thedeposition of the tungsten nucleation layer to the treatment operationcomprises turning on a flow of hydrogen.
 13. The method of claim 1wherein the resistivity of the tungsten film at 600 Angstroms is no morethan about 11 μΩ-cm.
 14. The method of claim 1 wherein the deposition ofthe tungsten nucleation layer is performed at a substrate temperature ofbetween about 250-325° C. with hydrogen flowed during or between thepulses of the tungsten-containing precursor and the reducing agent. 15.The method of claim 1 wherein the reducing agent is silane.
 16. A methodof forming a tungsten film on a substrate in a reaction chamber, themethod comprising: forming a tungsten nucleation layer on the substrateby alternating pulses of a boron-containing reducing agent and atungsten containing precursor at a substrate temperature of betweenabout 250-350° C., wherein no hydrogen is flowed during or between thepulses; performing a treatment operation on the deposited tungstennucleation layer, wherein the treatment operation comprises exposing thetungsten nucleation layer to alternating pulses of a borane and atungsten-containing precursor, wherein substantially no tungsten isdeposited on the tungsten nucleation layer during said treatmentoperation; and depositing a tungsten bulk layer over the treatedtungsten nucleation layer to form the tungsten film.
 17. The method ofclaim 16 wherein the resistivity of the tungsten film at 600 Angstromsis no more than about 11 μΩ-cm.
 18. A method of forming a tungsten layerin a small width feature, comprising: positioning a substrate having arecessed feature in a deposition station within a deposition chamber;forming a conformal tungsten nucleation layer in at least the feature byalternating pulses of a reducing agent and a tungsten-containingprecursor at a substrate temperature of between about 250-350° C.;performing a treatment operation on the deposited tungsten nucleationlayer, wherein the treatment operation comprises exposing the tungstennucleation layer to multiple pulses of a boron-containing reducing agentat a substrate temperature of at least about 350° C. whereinsubstantially no tungsten is deposited on the tungsten nucleation layerduring said treatment operation; and substantially filling the featurewith a tungsten bulk layer by exposing the substrate to atungsten-containing precursor and hydrogen at a substrate temperature ofat least about 350° C. to thereby deposit tungsten by a chemical vapordeposition process in at least the feature.
 19. The method of claim 18wherein forming a conformal tungsten nucleation layer comprisesalternating pulses of a boron-containing reducing agent and a tungstencontaining precursor, wherein no hydrogen is flowed during or betweenpulses.
 20. The method of claim 19, wherein the multiple pulses of theboron-containing reducing agent during the nucleation layer treatmentare performed in the presence of hydrogen and transitioning from thenucleation layer deposition to nucleation layer treatment comprisesturning on a flow of hydrogen.
 21. The method of claim 19, wherein theexposing the substrate to multiple pulses of boron-containing reducingagent comprises exposing the substrate to alternating pulses ofboron-containing reducing agent and a tungsten-containing precursor. 22.The method of claim 19 wherein the substrate temperature during thetreatment operation is at least about 375° C.
 23. An apparatus fordepositing tungsten film on a substrate comprising: a) a multistationsubstrate deposition chamber comprising: i) a tungsten nucleation layerdeposition station, the deposition station comprising a substratesupport and one or more gas inlets configured to expose the substrate topulses of gas; ii) a treatment station, the reducing agent exposurestation comprising a substrate support and one or more gas inletsconfigured to expose the substrate to pulses of gas; and b) a controllerfor controlling the operations in the multistation deposition chamber,the controller comprising instructions for: i) pulsing alternating dosesof boron-containing reducing agent and a tungsten containing precursorat a substrate temperature of between about 250-350° C., wherein nohydrogen is flowed during or between the pulses in the tungstendeposition station; ii) pulsing multiple doses of a boron-containingreducing agent at a substrate temperature of at least about 350° C. inthe treatment station and inletting hydrogen into the treatment stationduring said pulsing of multiple doses.